The present invention relates to an image forming apparatus for color printing by overlapping a plurality of unicolor images sequentially.
In the prior art, there has been developed an image forming apparatus such as a printer and a multifunction device capable of printing at varying printing magnification factors on the recording medium for printing such as paper. For example, Patent Document 1 discloses a technique for forming an image in response to a desired printing magnification factor by adjusting the scanning velocity in a sub-scanning direction on printing paper.
[Patent Document 1]
Official Gazette of Japanese Patent Tokkai 2002-305653
The aforementioned prior art, however, has the following problems: When color printing is carried out by forming a color image by overlapping a plurality of unicolor images (e.g. yellow, magenta, cyan, black, etc.), the scanning velocity in the sub-scanning direction will be changed if the printing magnification factor is modified. This will lead to misregistration in the printed positions of various colors resulting from modification of the scanning velocity.
For example, when color printing is applied to both sides of one printing paper, the temperature of printing paper in reverse side printing mode rises over that in the obverse side printing if printing is applied on the reverse side subsequent to printing on the obverse side. This produces a change in the size of the printing paper. This requires the printing magnification factor for the reverse side to be modified in duplex printing mode. In such a case, even if the optimum printing position is set for the obverse side, color drift will occur on the reverse side where printing magnification factor has been modified.
As described above, in color printing mode, modification of the printing magnification factor (especially when printing on the reverse side in duplex printing) leads to unexpected color drift, and causes deterioration in color printing image quality.